The Modulatory Action of Dopamine on Crustacean Foregut Neuromuscular Preparations

1. Effects of dopamine (DA) on contractile and electrical properties of decapod foregut neuromuscular preparations were examined. 2. Dopamine produced dramatic increases in nerve-evoked contractions and, in particular muscles, contractures and spontaneous contractions. These effects were observed at concentrations as low as 5 x io~~ M-DA. 3. The DA-produced enhancement of nerve-evoked contractions was associated with an increase in amplitude of excitatory junctional potentials (EJPs). 4. The increase in EJPs resulted in part from an increase in muscle fibre membrane resistance that was particularly prominent over depolarized membrane potentials. A presynaptic action of dopamine cannot be as yet excluded. 5. In fibres from the muscle in which dopamine produced a contracture, dopamine also produced a depolarization. 6. In fibres from the muscle in which dopamine activated spontaneous contractions, dopamine also produced spontaneous rhythmic action potentials. 7. Dopamine also accelerated the half-time of muscle relaxation following muscle contraction.